This invention relates to a flat lapping machine with an ultrasonic sizing mechanism that automatically laps the work to the preset desired thickness.
As prior art, FIG. 6 shows a known type of flat lapping machine that laps the work, such as a semiconductor wafer and magnetic disk substrate, with two surface plates at the top and bottom. As illustrated, an ultrasonic transducer 1 is affixed through a holder 2 to the top surface plate 3 in a position opposite to the work 4. Sizing is done on the basis of the thickness of the work 4 determined from the time passed before the echo of pulses of ultrasonic waves sent out from the transducer 1 to the work 4 is received.
In this known type of flat lapping machine, the transducer 1 is affixed in one of the slurried-abrasive feed holes provided in the top surface plate 3 by means of the holder 2. The transducer 1 is somewhat recessed from the bottom surface of the top surface plate 3, with provision being made to fill a space 5 left between the transducer 1 and the work 4 with slurried abrasive that serves as a propagation medium for ultrasonic waves fed through an inlet 7. Particles of the abrasive contained in the slurry, however, often disturb the waveform of the ultrasonic echo to such an extent as to produce errors in thickness measurement, preventing the achievement of high-precision sizing.
A known solution for the above problem is to affix the transducer in a position different from the slurried-abrasive feed hole using the same method as the one shown in FIG. 6. To ensure that the echo waveform is not disturbed, water containing no abrasive particles is fed through the inlet 7 as the propagation medium. This method, known as the water-immersion method, is one of basic ultrasonic flow detecting techniques offering an advantage of high-precision measurement because of the undistrubed echo waveform.
If this technique is applied just as it stands to the sizing in flat lapping machines, however, some portion of the slurried abrasive may get diluted because of the localized feed of water, which may in turn cause non-uniform lapping of the work, seriously impairing the finishing accuracy of the work.